In free space, electrons can move in any of three dimensions, based on the influence of electric and/or magnetic fields. Techniques of constructing semiconductor devices, which are well-known to one skilled in the semiconductor fabrication arts, can be used to control the flow of electrons so that their movement is confined to two dimensions, i.e., in a plane, or one dimension, i.e., along a line. These techniques include, e.g., heteroepitaxial materials growth (for 2-dimensional confinement), or growth and etching for one-dimensional confinement.
When electrons in the conduction band of silicon recombine with holes in the valence band, they lose energy, which may be given off as radiation. This process, however, is so inefficient that silicon cannot usefully be employed for radiative emission. The problem stems from silicon's indirect band gap, which requires the energy produced in the recombination process to be dissipated in ways other than radiation. Thus, any emission from normal silicon has negligible quantum efficiency. Silicon's observed unsuitability as a material for optical applications is a technological drawback since most other electronic functions can be performed using well-developed silicon technology. Thus, silicon would certainly be a material of choice if optical components designed for large-scale optoelectronic integration, e.g., in optical fiber communications systems, are to be made at reasonable cost.
Fortunately, quantum theory teaches that electrons which are constrained within a space comparable in size to the atomic dimensions of the material can lose energy and emit radiation in a process that is efficient enough to be useful. Such "quantum confinement" changes the band gap so that it becomes direct, i.e., all of the energy given off in recombination is available for radiation. The wavelength of the resulting energy emissions depends on the actual size of the confining space, and may be in the visible region of the electromagnetic spectrum.
The wavelength of the emitted radiation will vary in accordance with the cross-section of the one-dimensional channel in which the electron flow is confined. For example, it has been observed that the energy and wavelength of emitted radiation can be tuned by changing the thickness of very thin layers of silicon films sandwiched between silicon dioxide insulating layers. When electron waves are confined to a thin film, they are restricted to an integer number of oscillations within the film. As the film gets thinner, the corresponding wavelength becomes shorter, and electrons oscillate at higher energy levels as they move from the valence band to the conduction band, thus affecting the wavelength of energy emitted during recombination.